Method for manufacturing porous microstructures, porous microstructures manufactured according to this method, and the use thereof

ABSTRACT

A method for manufacturing porous microstructures in a silicon semiconductor substrate, porous microstructures manufactured according to this method, and the use thereof.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing porousmicrostructures, porous microstructures manufactured according to thismethod, and the use thereof.

BACKGROUND INFORMATION

Porous silicon has numerous possible uses in medicine due to a largeinternal surface, for example as a reservoir for the release ofmedications. Electrochemically manufactured porous silicon is typicallyused, a porous, spongy structure being produced in the silicon, forexample in the area of the surface of a silicon wafer.

Conventionally, porous silicon particles are manufactured according tothe method in which, for example, a large-area porous layer is producedon a silicon wafer, for example, this layer being detached by mechanicalaction, and the particles are separated by mechanical disintegration ofthe porous material, for example, by grinding.

Particles having undetermined size distribution and shape of theindividual particles arise through the mechanical disintegration. Inaddition, the surface of the porous particles is typically greatlydamaged or even destroyed by the mechanical action. In addition, theshape of the particles manufactured in this way is typically notreproducible, in particular, reproducible round particles may hardly bemanufactured in this way.

SUMMARY

An example method according to the present invention for manufacturingporous microstructures in a silicon semiconductor substrate may have theadvantage that porous silicon structures having previously defined shapeand size may be manufactured.

This may be achieved according to an example method of the presentinvention which includes the following steps:

-   a) applying and structuring a masking layer on the external surface    of the front side of a silicon semiconductor substrate, discrete    holes having a mean diameter in the range of ≧0.1 μm to ≦500 μm    being formed in the masking layer,-   b) producing recesses in a silicon semiconductor substrate, the    recesses each defining a substructure of the porous microstructures;-   c) removing the masking layer;-   d) applying and structuring a sacrificial layer, the sacrificial    layer having discrete through holes having a diameter in the range    of ≧0.1 μm to ≦150 μm, which are each situated centrally in the    recesses;-   e) applying a silicon layer, the silicon layer filling up the    recesses and forming a layer above the sacrificial layer;-   f) porosifying the silicon layer;-   g) applying and structuring a further masking layer on the    porosified silicon layer, the further masking layer covering the    areas above the recesses, which form a further substructure of the    porous microstructures;-   h) anisotropic etching of the porosified silicon layer;-   i) removing the further masking layer;-   j) removing the sacrificial layer;-   k) detaching or separating the porous microstructures.

The term “sacrificial layer” means a layer which is removed again in alater method step.

The term “doping” has the significance in the meaning of the presentinvention that the doped area or the doped layer has a higher dopingthan the original area or an adjoining area or layer of thesemiconductor substrate. Doping may be performed using boron, forexample, in particular, boron may be implanted or a boron glass coatingmay be performed.

Furthermore, manufacturing round, porous silicon microstructures havingdiameters from several micrometers up to several hundred micrometers, aswell as porous microneedles, is made possible by the method according tothe present invention.

A further advantage of the example method according to the presentinvention may be provided in that the method only has a few methodsteps. The example method according to the present invention preferablyrequires no more than three masking levels. This allows themanufacturing of porous microstructures having lower manufacturingcosts.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are shown in the drawingsand are explained in greater detail below.

FIGS. 1 a through 1 f show schematic cross-sectional views of the basicmanufacturing steps of porous microstructures in the form ofmicroneedles according to a first specific embodiment of the presentinvention.

FIGS. 2 a through 2 f show schematic cross-sectional views of theessential manufacturing steps of rounded porous microstructuresaccording to a second specific embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

An example method according to the present invention is first describedfor exemplary purposes.

A possible manufacturing method of porous microstructures 8 in the formof microneedles is shown in FIGS. 1 a through 1 f.

FIG. 1 a shows a silicon semiconductor substrate 1, which has recesses2. To manufacture recesses 2, a masking layer, for example formed fromSiO₂ or Si₃N₄, was applied to the external surface of the front side ofsilicon semiconductor substrate 1 and structured by photolithography.For this purpose, a photoresist layer, for example AZ® 1529 (AZElectronic Materials), was applied to the masking material, exposedusing a mask, and subsequently removed. The masking layer wassubsequently etched using reactive ion etching methods, for example,discrete holes corresponding to the exposure mask being formed in themasking layer.

To produce recesses 2, for example, an etching depth of 20 μm wasproduced using a self-stopping square mask opening of 28 μm edge lengthin an aqueous KOH solution at 80° C. Typical etching rates are in therange ≦4 μm/minute. Recesses 2 each define a substructure of porousmicrostructures 8. The masking layer was subsequently removed.

As shown in FIG. 1 b, a sacrificial layer 3 was applied to siliconsemiconductor substrate 1 having recesses 2. For this purpose, a silicondioxide layer of a thickness of approximately 1 μm was produced at atemperature of approximately 1100° C., preferably in a moistenvironment, for 1 to 2 hours through thermal oxidation of siliconsemiconductor substrate 1. Sacrificial layer 3 was structured bylithography. Holes 4 situated centrally in recesses 2 in the silicondioxide layer having a diameter of 8 μm were etched by etching using abuffered hydrofluoric acid solution containing 18.5 vol. % HF, inrelation to the total volume of the solution, for 15 minutes at 25° C.

In a following method step, a polycrystalline silicon layer 5 wasapplied using a PECVD method, as shown in FIG. 1 c, for example.Polycrystalline silicon layer 5 had a thickness above the recesses inthe range of 0.5 μm to 5 μm. Silicon layer 5 was subsequently p++ dopedby implantation of boron, doping having 10¹⁹ charge carriers/cm³ beingperformed. Doped silicon layer 5 was porosified by electrochemicaletching in an electrolyte containing hydrofluoric acid, having ahydrofluoric acid content in the range of 30 vol. % and having a contentof isopropanol or ethanol in the range of 10 vol. % to 30 vol. %, inrelation to the total volume of the electrolyte, the current densitybeing 100 mA/cm². In this case, a porosity of 40% to 50% and a porediameter of 20 nm were produced. The current preferably flowed throughholes 4 in sacrificial layer 3, whereby areas 6 in silicon layer 5 abovethe recesses were preferentially porosified, as shown in FIG. 1 d.

A masking layer, preferably photoresist, was applied to porosifiedsilicon layer 5 and structured by photolithography in a further methodstep, the masking layer covering the porosified areas of silicon layer 5above recesses 2. These areas form a further substructure of porousmicrostructures 8. The structures of porous microstructures 8 wereetched out of porosified silicon layer 5 by subsequent anisotropicetching, preferably using DRIE methods, as shown in FIG. 1 e. Theuncovered areas of silicon layer 5 were removed up to the etch stop onsilicon oxide sacrificial layer 3, whereby structures 7 of porousmicrostructures 8 protruding out of sacrificial layer 3 were formed.

Subsequently, sacrificial layer 3 was removed by currentless gas phaseetching employing HF vapor. Porous silicon structures 8 weremechanically removed by ultrasound. After the detachment from siliconsemiconductor substrate 1, porous microstructures 8 in the form ofmicroneedles were obtained, as shown in FIG. 1 f.

As shown in FIGS. 2 a through 2 f, the method according to the presentinvention is further usable to produce porous microstructures havingrounded shape 8′. Rounded recesses 2′ were first produced in a siliconsemiconductor substrate 1 by isotropic etching, for example using ClF₃.

To manufacture recesses 2′, a masking layer, for example formed fromSiO₂ or Si₃N₄, was applied to the external surface of the front side ofsilicon semiconductor substrate 1 and structured by photolithography.For this purpose, a photoresist layer, for example AZ® 1529 (AZElectronic Materials), was applied to the masking material, exposedusing a mask, and subsequently removed. The masking layer wassubsequently etched using reactive ion etching methods, for example,discrete holes corresponding to the exposure mask being formed in themasking layer.

For example, to produce recesses 2′ through holes in the masking layerhaving a diameter of 1 μm, an etching depth of 30 μm was produced at anetching rate of 10 μm/minute in an etching time of 3 minutes using ClF₃.This etching process is very isotropic, whereby a semi-round shape maybe obtained.

Typical etching rates are in the range ≦20 μm/minute. Recesses 2′ eachdefine a substructure of porous microstructures 8′. The masking layerwas subsequently removed.

As shown in FIG. 2 b, a sacrificial layer 3 was applied to siliconsemiconductor substrate 1 having recesses 2′. For this purpose, asilicon dioxide layer of a thickness of approximately 1 μm was producedat a temperature of approximately 1100° C., preferably in a moistenvironment, for 1 to 2 hours through thermal oxidation of siliconsemiconductor substrate 1. Sacrificial layer 3 was structured bylithography. Holes 4′ situated centrally in recesses 2′ in the silicondioxide layer having a diameter of 5 μm were etched by etching using abuffered hydrofluoric acid solution containing 18.5 vol. % HF, inrelation to the total volume of the solution, for 15 minutes at 25° C.

In a following method step, a polycrystalline silicon layer 5 wasapplied using a PECVD method, as shown in FIG. 2 c, for example.Polycrystalline silicon layer 5 had a thickness above the recesses inthe range of 0.5 μm to 5 μm. Silicon layer 5 was subsequently p++ dopedusing implantation of boron, a doping having 10¹⁹ charge carriers/cm³being performed. Doped silicon layer 5 was porosified by electrochemicaletching in an electrolyte containing hydrofluoric acid, having ahydrofluoric acid content in the range of 30 vol. % and having a contentof isopropanol or ethanol in the range of 10 vol. % to 30 vol. %, inrelation to the total volume of the electrolyte, the current densitybeing 100 mA/cm². In this case, a porosity of 40% to 50% and a porediameter of 20 nm were produced. The current preferably flowed throughholes 4′ in sacrificial layer 3, whereby areas 6′ in silicon layer 5above the recesses were preferentially porosified, as shown in FIG. 2 d.

A masking layer, preferably photoresist, was applied to porosifiedsilicon layer 5 and structured by photolithography in a further methodstep, the masking layer covering the porosified areas of silicon layer 5above recesses 2′. These areas form a further substructure of porousmicrostructures 8′. The structures of porous microstructures 8′ wereetched out of porosified silicon layer 5 by subsequent anisotropicetching, preferably using DRIE methods, as shown in FIG. 2 e. Theuncovered areas of silicon layer 5 were removed up to the etch stop onsilicon oxide sacrificial layer 3, whereby structures 7 of porousmicrostructures 8′ protruding out of sacrificial layer 3 were formed.

Subsequently, sacrificial layer 3 was removed by currentless gas phaseetching employing HF vapor. Porous silicon structures 8′ weremechanically removed by ultrasound.

After the detachment, porous microstructures 8′ having a rounded formwere obtained, as shown in FIG. 2 f.

It may be provided that before the removal of sacrificial layer 3,silicon semiconductor substrate 1 is immersed for a short time in anaqueous KOH solution at a temperature of 80° C., for example, structures7 of porosified silicon layer 5 protruding from sacrificial layer 3being rounded by the etching agent. In this way, nearly round poroussilicon structures having a diameter of 5 μm, for example, may beobtained.

Silicon wafers are usable as a particularly suitable siliconsemiconductor substrate.

As per the example method according to the present invention, recessesare produced in the silicon semiconductor substrate, the recesses eachdefining a substructure of the porous microstructures.

For this purpose, a first masking layer is applied to the externalsurface of the front side of a silicon semiconductor substrate andstructured, discrete holes having a mean diameter in the range of ≧0.1μm to ≦500 μm being formed in the first masking layer. In the context ofthe present invention, the term “hole” means an area of a layer in whichthe layer has a through opening, which exposes the external surface ofthe layer lying underneath. For example, the term “hole” means an areaof the masking layer in which the masking layer has a through opening,which exposes the external surface of the silicon semiconductorsubstrate. These holes allow the etching agent access to the siliconsemiconductor substrate. In the context of the present invention, theterm “discrete” means that the individual holes are not connected to oneanother. The holes are preferably spaced apart uniformly.

Preferably usable masking materials are SiO₂ or Si₃N₄ layers. Themasking layer may also be implemented from other substances, such asSiC. In the scope of the method according to the present invention,layers applicable using CVD (chemical vapor deposition), for examplesilicon oxide layers or suitable resist layers, are also usable as themasking layer.

The masking materials, in particular SiO₂ or Si₃N₄, are preferablystructured by photolithography. Photoresist layers having positive ornegative exposure properties are preferably usable, which may preferablybe structured thereafter using lithographic, in particularphotolithographic, methods. For example, liquid resist layers such asphotoresist are suitable. Photoresists obtainable under the name AZ®1529 (AZ Electronic Materials) are usable, for example. The photoresistlayer may be exposed using a mask, such as a silicon dioxide layer, andsubsequently removed.

The masking layer, in particular a SiO₂ or Si₃N₄ layer, is preferablyetched using suitable etching agents, discrete holes having a meandiameter in the range of ≧0.1 μm to ≦500 μm being formed in the maskinglayer.

The holes in the masking layer preferably have a round shape.Furthermore, the holes in the masking layer may have a polygonal shape,for example, the holes in the masking layer may be square. The meandiameter of the holes of the masking layer is in the range of ≧0.1 μm to≦100 μm, more preferably in the range of ≧0.1 μm to ≦50 μm, still morepreferably in the range of ≧1 μm to ≦10 μm in preferred specificembodiments.

The recesses in the silicon semiconductor layer may have a round,rounded, or oval shape, for example. Furthermore, the recesses may havea polygonal or pointed shape. In further preferred specific embodiments,the recesses have a pointed shape, for example the form of a polygonsuch as a tetrahedron. It may be preferable for recesses to taper indepth, run to a point, and/or end in a point. It is advantageous thatspecific embodiments having pointed recesses may each define asubstructure of porous microstructures, which have the form of amicroneedle.

In preferred specific embodiments, the recesses have a rounded shape, inparticular the form of a semi-sphere or generally the form of asemi-sphere. In advantageous specific embodiments of the methodaccording to the present invention, rounded recesses are produced byisotropic etching. Preferred etching agents for isotropic etching areselected from the group including ClF₃, BrF₃, XeF₂, and/or SF₆.Furthermore, other media which isotropically etch silicon, such asmixtures of HNO₃ with H₂O and NH₄F and/or mixtures of theabove-mentioned etching agents are suitable.

It is advantageous that specific embodiments having rounded recesses, inparticular in the form of a semi-sphere or generally the form of asemi-sphere, may each define a substructure of porous microstructureswhich have a round form.

The dimensions of the porous microstructures may thus advantageously bedefined during the manufacturing of the microstructures.

In further preferred specific embodiments of the method according to thepresent invention, pointed or polygonal recesses are produced byanisotropic etching. Preferred anisotropic etching methods are, forexample, DRIE (deep reactive ion etching) methods or etching methodsemploying KOH and/or tetramethyl ammonium hydroxide (TMAH) solutions.

The mean diameter of the produced recesses is in the range of ≧0.1 μm to≦500 μm, preferably in the range of ≧0.1 μm to ≦100 μm, more preferablyin the range of ≧1 μm to ≦50 μm in preferred specific embodiments.

The depth of the recesses may be in the range of several micrometers upto several hundred micrometers, according to the intended use of themicrostructures. The depth of the recesses may make up approximatelyhalf of the diameter of the recesses in the case of recesses of roundedshape in particular. In other specific embodiments, the depth of therecesses may be in the range of the diameter of the recesses or may bedeeper, for example in the range of two to five times the length. Thisis advantageous in particular if microstructures having a tip are to beproduced, for example in the form of microneedles. In preferred specificembodiments of the method according to the present invention, recessesare produced having a depth in the range of ≧1 μm to ≦500 μm, morepreferably in the range of ≧1 μm to ≦100 μm, still more preferably inthe range of ≧1 μm to ≦50 μm.

In a following method step, a sacrificial layer is applied andstructured, the sacrificial layer having discrete through holes having adiameter in the range of ≧0.1 μm to ≦150 μm, which are each situatedcentrally in the recesses.

The sacrificial layer preferably includes a silicon oxide layer, and thesacrificial layer is preferably implemented from SiO₂. For example, asilicon oxide layer may be applied by PECVD (plasma-enhanced chemicalvapor deposition) methods. A silicon oxide layer is preferably producedby thermal oxidation. A thermal oxide, which is preferably formed fromsilicon oxide, may be grown on the silicon semiconductor substrate bythermal oxidation.

The thickness of the sacrificial layer is preferably in the range of≧100 nm to ≦5 μm, more preferably in the range of ≧0.5 μm to ≦4 μm,particularly preferably in the range of ≧0.5 μm to ≦2 μm.

The sacrificial layer is preferably structured by photolithography. Theetching of the sacrificial layer is preferably performed using bufferedoxide etching (BOE), preferably employing hydrofluoric acid which isbuffered using ammonium fluoride, in particular a HF/NH₄F/H₂O solution.It is advantageous in the case of buffered oxide etching that thesacrificial layer is removable without damaging the silicon layers lyingunderneath.

In preferred specific embodiments of the method according to the presentinvention, the discrete through holes of the sacrificial layer have adiameter in the range of ≧0.1 μm to ≦100 μm, preferably in the range of≧0.1 μm to ≦50 μm, more preferably in the range of ≧0.5 μm to ≦50 μm,still more preferably in the range of ≧1 μm to ≦20 μm, particularlypreferably in the range of ≧1 μm to ≦10 μm.

The discrete through holes of the sacrificial layer advantageously allowthe current to flow through the holes of the sacrificial layer in alater step of porosification. This provides the advantage that thecurrent may be preferentially conducted to the target structures andthey may be preferably etched in a porous manner. A further greatadvantage which is provided by the holes in the sacrificial layer isthat they are filled up by silicon in the further course of the methodand may be used as support points and/or predetermined break points ofthe microstructures on the semiconductor substrate.

In a following method step, silicon is deposited on the sacrificiallayer. For this purpose, silicon is deposited in the recesses and asilicon layer is applied. The recesses are preferably completely filledby the deposited silicon. Furthermore, a preferably contiguous siliconlayer is applied over it. Both polycrystalline and also monocrystallinelayers are suitable, a polycrystalline layer being preferred. Apolycrystalline silicon layer may preferably be applied by PECVD(plasma-enhanced chemical vapor deposition) methods. Furthermore, asilicon layer may be applied by PECVD methods, on which an epitacticallydeposited polycrystalline silicon layer may be applied.

In preferred specific embodiments of the method, a silicon layer havinga thickness above the recesses in the range of ≧1 μm to ≦100 μm,preferably in the range of ≧2 μm to ≦50 μm, more preferably in the rangeof ≧5 μm to ≦20 μm is applied.

The thickness of the silicon layer above the recesses may correspond tothe depth of the recesses and/or approximately half of the diameter ofthe recesses, in particular in the case of microstructures to bemanufactured having a round or an essentially round shape. In otherspecific embodiments, the thickness of the silicon layer may be in therange of the diameter of the recesses or less. This is advantageous inparticular if microstructures are to be produced in the form ofmicroneedles.

In preferred specific embodiments of the method, the silicon layer isplanarized and/or doped.

In preferred specific embodiments, the silicon layer may be planarized.A preferred method of planarization is polishing. In preferred specificembodiments, the silicon layer may be polished, preferably using CMP(chemical-mechanical polishing), in which the abrasive action of agrained polishing head is supported by the chemical action of a suitablesolution. The surface of the microstructure may be defined and shaped ina defined method thereafter by the planarization, in particularpolishing, because the topography which results through the applicationof the silicon layer to the recesses may be leveled by this step.

In further preferred specific embodiments, the silicon layer may bedoped. Doping is preferably performed using boron; in particular, boronmay be implanted or a boron glass coating may be performed. It may bepreferable for the applied silicon layer to be doped. In other specificembodiments, it may be preferable to introduce the doping into thematerial during the application of the silicon layer.

The pore structure may advantageously be influenced by the selection ofthe doping. Doping in the range of up to 10¹⁹/cm³ may be used, theparameter corresponding to the number of doping atoms per cm³ of thesilicon semiconductor substrate. A mesoporous structure may be achievedby a doping having 10¹⁹/cm³, whose pore diameter is preferably in therange of ≧2 nm to ≦100 nm. The advantage of a mesoporous structure ofthe porosity of the microstructures is in particular that substances ordrugs which are to be introduced into a body, for example, may be easilyintroduced into mesoporous structures.

Preferably, porosification is performed in electrolytes containinghydrofluoric acid, in particular aqueous hydrofluoric acid solutions, ormixtures containing hydrofluoric acid, water, and further reagents, inparticular selected from the group including wetting agents such asalcohols, preferably selected from the group including ethanol and/orisopropanol, and/or tension-reducing agents such as surfactants. Forexample, ethanol and/or isopropanol are usable in the range of ≧10 vol.% to ≦30 vol. %, in relation to the total volume of the electrolyte.

The hydrofluoric acid content of an aqueous hydrofluoric acid solutionis preferably in the range of ≧10 vol. % to ≦40 vol. %, in relation tothe total volume of the electrolyte. A wetting agent may be added.Preferred wetting agents are selected from the group includingisopropanol and/or ethanol.

Preferred current densities are in the range of ≧10 mA/cm² to ≦200mA/cm², preferably in the range between ≧50 mA/cm² and ≦150 mA/cm².

A special advantage of the method according to the present invention maybe provided in that the current may preferably flow through the holes inthe sacrificial layer and is preferentially conducted to the structuresto be manufactured. In this way, they are preferably etched to becomeporous.

The porosity is settable by suitable selection of the processingparameters, such as the electrolyte composition, in particular thehydrofluoric acid concentration, and/or the current density. Theporosity specifies the ratio of the empty space inside a structure andthe remaining semiconductor substrate or silicon material. A porosity ofthe porous microstructures in a range of approximately 10% to greaterthan 90% is thus preferably settable, preferably in the range of ≧10% to≦80%, particularly preferably in the range of ≧20% to ≦70%.

In preferred specific embodiments of the method, a porosity of themicrostructure is set in a range of ≧10% to ≦90%.

“Porosity” is defined in the meaning of the present invention so that itspecifies the empty space inside the structure and the remainingsubstrate material. It may either be optically determined, i.e., fromthe analysis of microscopic pictures, for example, or chemically. In thecase of chemical determination, the following equation applies: porosityP=(m1-m2)/(m1-m3), m1 being the mass of the sample before theporosification, m2 being the mass of the sample after theporosification, and m3 being the mass of the sample after etching using1 molar sodium hydroxide solution, which chemically dissolves the porousstructure. Alternatively, the porous structure may also be dissolved bya KOH/isopropanol solution.

Various pore structures are producible depending on the processingparameters; thus, nanopores, mesopores, or macropores may be generated.The pore size may be set in a range of several nanometers up to 50 nmdiameter depending on the hydrofluoric acid concentration, doping, andcurrent density. For example, pores having a diameter ≦5 nm, in therange between ≧5 nm and ≦50 nm, or ≧50 nm may be manufactured. Inpreferred specific embodiments, pores having a diameter in the range of≧1 nm to ≦1 μm, preferably in the range of ≧2 nm to ≦100 nm, morepreferably in the range of ≧5 nm to ≦30 nm may be manufactured.

In a further method step, a further masking layer is applied andstructured on the porosified silicon layer, the further masking layercovering the areas above the recesses, which form a further substructureof the porous microstructures. In this way, the lateral structure of themicroparticles to be manufactured may advantageously be formed.

A photoresist layer having positive or negative exposure properties ispreferably used for this purpose, which is subsequently preferablystructured using photolithographic methods. For example, liquid resistlacquers such as photoresist are suitable. The photoresist layer may beexposed using a mask, such as a chromium layer on a quartz substrate.

The areas above the recesses, which the masking layer covers, preferablyhave a diameter corresponding to the diameter of the recesses and/or ashape corresponding to the shape of the recesses.

In a further method step, the porosified silicon layer isanisotropically etched. For this purpose, the areas of the porosifiedsilicon layer above the recesses, which are protected by the maskinglayer, remain. The structures remaining after the etching form a furthersubstructure of the porous microstructures. The sacrificial layer belowthe porosified silicon layer is not etched by the anisotropic etching.Preferred methods are, in particular, DRIE (deep reactive ion etching)methods. A preferred etching agent is SF₆ in this case.

The masking layer may be removed after the etching.

In an optional method step, the areas of the porosified silicon layerabove the recesses which are protected by the masking layer, and whichform the further substructure of the porous microstructures, may berounded, preferably by isotropic etching.

In preferred specific embodiments of the method, the substructure of theporous microstructures protruding out of the sacrificial layer isrounded by isotropic etching.

In preferred specific embodiments, the rounding may be performed bybrief immersion in a KOH solution and/or a so-called HNA solutionincluding HF, preferably 1 vol. %, HNO₃, preferably 3 vol. %, andCH₃COOH, preferably 8 vol. %, each in relation to the total volume. Afurther preferred specific embodiment provides the introduction of themicrostructures into an atmosphere containing ClF₃. It is advantageousin particular that round or generally round porous siliconmicrostructures may be manufactured by rounding of the structures abovethe recesses. However, it may also be advantageous to round thestructure in the case of porous microstructures to be manufactured inthe form of microneedles.

In a further method step, the sacrificial layer is removed. The removalmay preferably be performed by etching in hydrofluoric acid (HF) vapor.This may provide the advantage that the porous microstructures are heldby the silicon areas inside the previous holes of the sacrificial layeron the silicon semiconductor substrate. The microstructures may thus befunctionalized on the semiconductor substrate in an optional methodstep.

In other preferred specific embodiments, the removal of the sacrificiallayer may be performed by etching in hydrofluoric acid (HF) solution.This may provide the advantage that the porous microstructures may bedetached from the silicon semiconductor substrate by the etching. Theporous microstructures may be obtained from the solution by filtering,for example.

In further specific embodiments having suitable sacrificial layers, forexample a layer system including oxide and silicon-germanium layers, thesacrificial layer may be removed by gas phase etching using etchingagents selected, for example, from the group including ClF₃ and/or XeF₂.

In preferred specific embodiments of the method, the porousmicrostructures are functionalized before the detachment from thesemiconductor substrate. For example, the porous microstructures on thesemiconductor substrate may be doped, impregnated, charged, or oxidizedbefore they are detached. The porous microstructures are preferablyimpregnated with active substances, such as pharmaceuticals. Thisprovides the advantage that uniform and controlled functionalization maybe performed on the semiconductor substrate.

The porous microstructures are preferably detached or separated bymechanical action, ultrasound, or by a flow of a compressed gas overthem. It is advantageous in this case that the porous microstructureswhich are held by support points may be detached by slight mechanicalaction.

It is particularly advantageous in this case that the semiconductorsubstrate, in particular a silicon wafer, does not have to be completelydestroyed in this way, but rather may be reused. For example, thesemiconductor substrate may be usable for manufacturing porousmicrostructures again after grinding and/or polishing. Furthermore, themethod according to the present invention is advantageous in that thefurther usability of the semiconductor substrate allows themanufacturing costs of porous microstructures made of silicon to bereduced.

A further object of the present invention relates to porousmicrostructures which are manufactured using the method according to thepresent invention.

The porous microstructures are fundamentally suitable for allapplications which require porous microstructures, in particular porousparticles or microneedles. In particular, the microstructures aresuitable for biological applications, because microstructures made ofporous silicon are biocompatible and may be resorbed by the body inparticular. In preferred uses of the porous microstructures, they may beused as a reservoir for the application of drugs or active substances ormay be used for the manufacture of application units for drugs ormedications. It is advantageous that the porous microstructures areusable as a reservoir for drugs or active substances. Furthermore, theporous structures may allow a painless application of drugs ormedications.

Furthermore, a use of the porous microstructures manufactured accordingto the present invention for injection into the bloodstream isadvantageous.

A preferred use is a localized application of medications. For example,porous microstructures charged with an angiogenesis inhibitor aresuitable for blocking blood vessels which supply a tumor. It isadvantageous in this case that the mechanical blocking action may belocally supported by the anti-angiogenic effect through the release ofthe substance.

For example, usable anti-angiogenic substances are selected from thegroup including bevacizumab, available under the trade name Avastin®from Genentech/Roche, vandetanib (ZD6474), available under the tradename Zactima® from Astra-Zeneca PLC, and/or the substancePTK787/ZK222584 (Novartis/Schering).

The porous microstructures may have an arbitrary form. The manufacturedporous microstructures are preferably round or essentially round or havethe form of a microneedle.

A particular advantage of the method according to the present inventionmay be provided in that round or generally round porous siliconmicrostructures may be manufactured. In preferred specific embodiments,round or generally round porous silicon microstructures may bemanufactured, which have a diameter in the range of ≧1 μm to ≦500 μm,more preferably in the range of ≧1 μm to ≦50 μm, particularly preferablyin the range of ≧1 μm to ≦5 μm. These porous silicon microstructures areusable in particular for injection into the bloodstream. It isadvantageous in this case that using round porous siliconmicrostructures having a diameter of less than 5 μm may have the resultthat vessels are not closed, or are only closed very slightly.

In further preferred specific embodiments, round or generally roundporous silicon microstructures may be manufactured, which have adiameter in the range of ≧1 μm to ≦500 μm, more preferably in the rangeof ≧1 μm to ≦50 μm, particularly preferably in the range of ≧10 μm to≦50 μm. These porous silicon microstructures may allow vessels to beintentionally closed. Furthermore, an accumulation of the porous siliconmicrostructures may be achieved. A possible application is anadministration of radioactive tracers, for example, which collect inorgans and thus allow an examination of the perfusion. A further exampleapplication is the targeted suppression of the blood flow, for example,in oncology.

The porous microstructures manufactured by the method according to thepresent invention preferably have a typical orientation of the pores.The pores which originate from the substrate surface in the porousmicrostructures are preferably oriented perpendicular to the pores whichoriginate from the recesses.

1-12. (canceled)
 13. A method for manufacturing a porous microstructurein a silicon semiconductor substrate, comprising: a) applying andstructuring a masking layer on an external surface of a front side of asilicon semiconductor substrate, discrete holes having a mean diameterin the range of ≧0.1 μm to ≦500 μm being formed in the masking layer; b)producing recesses in a silicon semiconductor substrate, the recesseseach defining a substructure of the porous microstructures; c) removingthe masking layer; d) applying and structuring a sacrificial layer, thesacrificial layer having discrete through holes having a diameter in therange of ≧0.1 μm to ≦150 μm, which are each situated centrally in therecesses; e) applying a silicon layer, the silicon layer filling up therecesses and forming a layer above the sacrificial layer; f) porosifyingthe silicon layer; g) applying and structuring a further masking layeron the porosified silicon layer, the further masking layer coveringareas above the recesses, which form a further substructure of theporous microstructures; h) anisotropic etching the porosified siliconlayer; i) removing the further masking layer; j) removing thesacrificial layer; and k) detaching or separating the porousmicrostructures.
 14. The method as recited in claim 13, wherein therecesses are rounded recesses, and are produced by isotropic etching.15. The method as recited in claim 13, wherein the recesses are one ofpointed or polygonal recesses, and are produced by anisotropic etching.16. The method as recited in claim 13, wherein the recesses have a depthin the range of ≧1 μm to ≦500 μm.
 17. The method as recited in claim 16,wherein the range is ≧1 μm to ≦100 μm.
 18. The method as recited inclaim 16, wherein the range is ≧1 μm to ≦50 μm.
 19. The method asrecited in claim 13, wherein the discrete through holes in thesacrificial layer have a diameter in the range of ≧0.1 μm to ≦50 μm. 20.The method as recited in claim 19, wherein the range is ≧1 μm to ≦20 μm.21. The method as recited in claim 13, wherein in step e), the siliconlayer having a thickness above the recesses in a range of ≧1 μm to ≦100μm.
 22. The method as recited in claim 21, wherein the range is ≧2 μm to≦50 μm.
 23. The method as recited in claim 21, wherein the range is ≧5μm to ≦20 μm.
 24. The method as recited in claim 13, wherein thesubstructure of the porous microstructures which protrudes from thesacrificial layer is rounded by isotropic etching.
 25. The method asrecited in claim 13, wherein the silicon layer is at least one ofplanarized and doped.
 26. The method as recited in claim 13, wherein aporosity of the microstructures is set in the range of ≧10% to ≦90%. 27.The method as recited in claim 13, wherein the porous microstructuresare functionalized before detachment.
 28. Porous microstructures, theporous microstructures being formed in a silicon semiconductor substrateby: a) applying and structuring a masking layer on an external surfaceof a front side of a silicon semiconductor substrate, discrete holeshaving a mean diameter in the range of ≧0.1 μm to ≦500 μm being formedin the masking layer; b) producing recesses in a silicon semiconductorsubstrate, the recesses each defining a substructure of the porousmicrostructures; c) removing the masking layer; d) applying andstructuring a sacrificial layer, the sacrificial layer having discretethrough holes having a diameter in the range of ≧0.1 μm to ≦150 μm,which are each situated centrally in the recesses; e) applying a siliconlayer, the silicon layer filling up the recesses and forming a layerabove the sacrificial layer; f) porosifying the silicon layer; g)applying and structuring a further masking layer on the porosifiedsilicon layer, the further masking layer covering areas above therecesses, which form a further substructure of the porousmicrostructures; h) anisotropic etching the porosified silicon layer; i)removing the further masking layer; j) removing the sacrificial layer;and k) detaching or separating the porous microstructures
 29. A methodof applying drugs, comprising: manufacturing a porous microstructure ina silicon semiconductor substrate, including: a) applying andstructuring a masking layer on an external surface of a front side of asilicon semiconductor substrate, discrete holes having a mean diameterin the range of ≧0.1 μm to ≦500 μm being formed in the masking layer, b)producing recesses in a silicon semiconductor substrate, the recesseseach defining a substructure of the porous microstructures, c) removingthe masking layer, d) applying and structuring a sacrificial layer, thesacrificial layer having discrete through holes having a diameter in therange of ≧0.1 μm to ≦150 μm, which are each situated centrally in therecesses, e) applying a silicon layer, the silicon layer filling up therecesses and forming a layer above the sacrificial layer, f) porosifyingthe silicon layer, g) applying and structuring a further masking layeron the porosified silicon layer, the further masking layer coveringareas above the recesses, which form a further substructure of theporous microstructures, h) anisotropic etching the porosified siliconlayer, i) removing the further masking layer, j) removing thesacrificial layer, and k) detaching or separating the porousmicrostructures, and applying drugs using the porous microstructure.